The dangerous wobbling of pedestrian bridges could be reduced by using biomechanically inspired models of pedestrian response to bridge motion and a mathematical formula to estimate the critical crowd size at which bridge wobbling begins, according to a study led by Georgia State University. Many pedestrian bridges around the world have experienced dramatic vibrations and dangerous wobbling when crowds of pedestrians have tried to cross them, with some bridges falling down. Bridges all over the world have experienced this including the Squibb Park Bridge in Brooklyn that remained closed for years after wobbling.
"In this Science Advances paper, we reveal this threshold effect and show this is a general phenomenon for bridges," said Dr. Igor Belykh, professor in the Department of Mathematics and Statistics at Georgia State. "We challenge the widespread view that increasing the crowd size will gradually increase the bridge wobble. The current view is the more pedestrians we add to the bridge, the wilder the oscillations will be. This is true, but only for crowd sizes above this critical size. There is an important threshold effect.
"Our paper gives an explicit guideline and formula of how to estimate this critical crowd size, which can be used to limit the carrying capacity of an existing bridge and to help designers build better bridges. The biomechanical models we're developing are particularly important for understanding the role of crowd dynamics on a wobbly bridge because the U.S. code for designing pedestrian bridges does not contain specific guidelines that account for collective pedestrian behavior. The industry standard programs used by bridge designers only use linear models. We're working on the inclusion of biomechanical models like ours into the standard tools and software programs used by bridge designers to better predict the nonlinear effects associated with the interaction between crowds of pedestrians and bridges."
In 2014, the Squibb Park Bridge in Brooklyn, N.Y. bounced from side to side as pedestrians crossed the bridge and didn't reopen until early 2017.